In, for example, genetic diagnoses of diseases and gene expression analysis, it is necessary to detect a nucleic acid having a specific sequence. For the detection, methods utilizing fluorescence are used widely, and for instance, a fluorescence probe obtained by covalently bonding one type of fluorescent dye to DNA often is used as a labeling substance.
Such a labeling substance (a fluorescence probe) has a problem in that, for example, it emits fluorescence even when it has not formed a double helix with a complementary nucleic acid. For the purpose of quenching fluorescence of only the probe, the method using fluorescence resonance energy transfer (FRET) is effective (e.g., Non-Patent Documents 1 to 4). However, it has problems in, for example, cost due to the introduction of two types of fluorescent dyes.
Thiazole orange, which is one type of cyanine dye, is known as a fluorescent dye whose fluorescence intensity increases through an interaction with DNA or RNA. There are examples in which a fluorescent probe was intended to be produced with thiazole orange being bonded to DNA by a covalent bond. However, it also emits strong fluorescence through an interaction with a single-stranded DNA containing a purine base (Non-Patent Document 5). Accordingly, the increase in fluorescence intensity obtained when a double helix is formed is small, and therefore it cannot be considered as being successful (Non-Patent Documents 6 and 7).    Non-Patent Document 1: Tyagi, S., Kramer, F. R. (1996) Nat. Biotechnol. 14, 303-308.    Non-Patent Document 2: Nazarenko, I. A., Bhatnagar, S. K., Hohman, R. J. (1997) Nucleic Acids Res. 25, 2516-2521.    Non-Patent Document 3: Gelmini, S., Orlando, C., Sestini, R., Vona, G., Pinzani, P., Ruocco, L., Pazzagli, M. (1997) Clin. Chem. 43, 752-758.    Non-Patent Document 4: Whitcombe, D., Theaker, J., Guy, S. P., Brown, T.,    Little, S. (1999) Nat. Biotechnol. 17, 804-807.    Non-Patent Document 5: Biopolymers 1998, 46, 39-51.    Non-Patent Document 6: Analytica Chimica Acta 2002, 470, 57-70.    Non-Patent Document 7: Chemistry—A European Journal 2006, 12, 2270-2281.